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    Home > Chem: synthesis of α - quaternary chiral aldehydes from styrene, allyl alcohol and H2 / Co

    Chem: synthesis of α - quaternary chiral aldehydes from styrene, allyl alcohol and H2 / Co

    • Last Update: 2018-04-13
    • Source: Internet
    • Author: User
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    The author of the paper: in recent years, the multi catalyst relay catalyst system (Figure 1), which is used by two or more kinds of catalysts together, has become a promising organic synthesis strategy

    In such a system, different types of catalysts (i.e

    metal catalysts, organic catalysts or enzyme catalysts, etc.) can achieve traditional multi-step synthesis in one operation cooperatively or independently, thus greatly reducing the waste of raw materials, solvents, time, etc

    Figure 1 concept of multi catalyst relay Catalysis (source: Chem) Although some achievements have been made in the field of relay catalysis, there are still some problems and arduous challenges in the multi catalyst system: (1) there are few reports on the multi catalyst system involving more than three different catalysts, because it may lead to compatibility problems of many catalysts and intermediates while realizing more complex transformation; (2) successful multi catalyst system, especially that Most of the multi catalyst systems need to use designed substrates for asymmetric series reactions, which need multi-step synthesis

    In response to the above problems and challenges, recently, Gong Liuzhu group of China University of science and technology realized the conversion of styrene, allyl alcohol and H 2 / CO to α - quaternary carbon functional aldehydes through the combination of RH catalytic hydroformylation and PD, organic catalyst CO catalytic asymmetric allylation

    Relevant articles were published on chem (DOI: 10.1016 / j.champr

    2018.03.010)

    Fig

    2 reaction route (source: Chem) transition metal catalyzed carbonylation and allylation (such as Tsuji trostrection) are classic reactions in organic synthesis

    The combination of these two important processes means that two reactions are carried out in one reactor at the same time, which greatly improves the synthesis efficiency

    In the author's hypothesis, olefins are selectively hydroformylated under the catalysis of RH, and then asymmetric α - allylation is carried out on aldehydes produced in situ under the catalysis of PD and amine, finally α - quaternary functionalized chiral aldehydes are produced

    List group reported the direct α - allylation of aldehydes and ketones catalyzed by Pd / organic catalysts (angel

    Chem

    Int

    ed

    2011, 50, 9471 – 9474)

    Gong Liuzhu group also reported the aminomethylation of olefins catalyzed by Rh / chiral Bronsted acid (org

    Lett

    2017, 19, 1076 – 1079)

    Inspired by the study of asymmetric α - allylation of aldehydes, the author explored the hypothesis

    Allyl alcohols were chosen as Allylating agents because they are easy to use

    The co catalyst system of PD and organic catalyst was chosen to promote the asymmetric α - allylation of aldehydes produced in situ

    As shown in Fig

    3, allyl alcohol is converted into p-allyl PD complexes with chiral anions in the presence of non chiral PD (0) catalyst and BINOL derived chiral phosphoric acid

    The Nonchiral amines react with α - branched aldehydes produced by hydroformylation of alkenes to form an intermediate of the amines

    The amines II react with the p-allyl Pd complex IV to form the chiral imines VI, which, after in-situ hydrolysis, produce α - quaternary carbochiral aldehydes 3 and release amines

    In this catalytic system, asymmetric hydroformylation is not necessary due to racemization caused by the formation of enamine

    Chiral phosphoric acid is the only chiral factor controlling the stereoselectivity in the whole reaction

    Fig

    3 reaction mechanism (source: Chem) at the beginning of the study, the author used the multi catalyst system containing Rh complex, Pd (PPh3) 3, chiral phosphoric acid (R) - trip and chiral amine A1, and styrene (1a), cinnamyl alcohol (2a) reacted in H 2 / CO atmosphere

    In order to reduce the side reactions caused by H 2 / CO and inhibit the deactivation effect of Pd catalyst caused by CO coordination, the author used 1 bar gas (CO / H 2 = 1:1), which is the key factor of the reaction

    The yield can be effectively increased by reducing the amount of Rh complex to 1 mol%

    In addition, the yield and stereoselectivity of the reaction were greatly improved by increasing the amount of amine A1 to 1 Eq

    (Fig

    4)

    Fig

    4 condition screening (source: Chem) after determining the optimal reaction conditions, the author expanded the substrate, and found that a series of cinnamic alcohols can get α - quaternary carbaldehyde smoothly, with medium to excellent yield (57% - 97%), and high to excellent enantioselectivity (86% - 99% ee, 3b – 3I)

    Figure 5 reactant screening (source: Chem) followed by the author's exploration of the versatility of styrene derivatives by the reaction of 2-phenylpropyl-2-ene-1-ol and styrene

    The enantioselectivity (84% - 99% ee, 3S – 3AA) of the chiral aldehydes was excellent

    Figure 6 reactant screening (source: Chem), corresponding author: Professor Gong Liuzhu, associate researcher Han Zhiyong
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